Turbomachines of all kinds are most efficient when they operate close to limits imposed by their materials, mechanical clearances and by flow instabilities. It is vitally important to monitor the dynamic stresses and other conditions of critical machine components during operation so that these limits can be approached with safety and reliability. The rotating blades of a turbomachine are its most highly stressed components, but it is difficult to measure the dynamic conditions of these blades because they are rotating.
The rotating blade conditions whose measurement would be valuable in monitoring and controlling a turbomachine arm (1) end or side clearance, (2) vibration and flutter, and (3) untwist. Rotating turbomachinery blades have been instrumented for all of these, but the sensors used to date have all been unsuitable for long term monitoring, or deficient in other respects.
Blade clearances may be measured statically or during turbomachine operation by conventional sensors of several types. For example, variable inductors and variable capacitors are mentioned by I. E. Zablotsky et al in U.S. Pat. No. 3,467,358, in a system for indicating vibrations in turbomachinery. These types of sensors are well adapted to the desired measurement, but their signals are subject to drift and have other known limitations. Variable inductance probes require that the turbomachinery blades be made of a ferrous or other magnetic material, and are limited in their application to a fraction of machines in service. Their frequency response is severely limited by their inductance. Variable capacitance probes are limited in the range of clearance distances they can transduce and are subject to long term degradation by particulate and chemical contaminants in the fluid stream. They also require low capacitance per unit length cabling and are sensitive to capacitance variations in the cables.
Conventional eddy current sensors may also be used for the measurement of clearance distances in a turbomachine. In their 1981 ISA paper, "Non-Contacting Method For Measurement of Dynamic Blade Motions in Axial-Flow Compressors", Walter F. O'Brien et al describe the use of commercial sensors purchased from the Bently-Nevada Corporation to measure blade clearances and vibrations in an experimental compressor fan rig. U.S. Pat. No. 4,518,917, issued to Robert M. Oates et al, describes similar sensors as part of a system for monitoring turbine blading.
Turbomachinery blade clearances have also been measured by using laser probes. For example, in U.S. Pat. No, 4,326,804, Paul W. Mossey describes a system for optical clearance distance determination, including purging means for keeping the optics clean. Such means are necessary because the optical probe must operate in a high speed stream containing corrosive gases and abrasive particulates. These are both likely to degrade the performance of optical elements such as mirrors and lenses.
In U.S. Pat. No. 4,384,819, Peter D. Baker discloses a turbomachinery blade clearance measuring system which uses microwave radiation. This system is well adapted to the environment of a turbomachine, but is intricate, expensive, and subject to drift.
Vibration, flutter and untwist can be measured in a turbomachine by any of the probes mentioned above. For example, U.S. Pat. No. 4,153,388 issued to John P. Naegeli describes a system which uses inductive pickups for detecting unwanted vibrations of turbomachinery blades. Any of the detection methods described so far can be used for this purpose because the timing of signals they produce is indicative of the dimensions of interest.
During the development of turbomachines it is common to mount strain gauges on the blades themselves and use slip rings coaxially mounted with the machine shaft to carry their signals to fixed instruments. However, this type of blade condition monitoring systm is expensive and requires components which can endure the internal conditions of the machine. It is not generally suitable for long term machine monitoring.
It has been learned that important information about incipient or actual flow instabilities may be derived from blade operating conditions. Rotating stall in a compressor, for example, may be detected by a system which measures blade vibration. Such a detection system makes it possible to operate the machine closer to the limit imposed by stall, where efficiency and performance are generally greater.
There are many instances in machine development and test when it would be valuable to have real-time measurements of dynamic blade operating conditions. While existing sensors have the capability of performing this function at ordinary operating speeds, the advanced machines under development operate at higher tip speeds and blade rates, smaller clearances and higher temperatures. These conditions make it difficult or impossible to use existing sensors to gather the needed information.
In U.S. Pat. No. 4,439,728, issued to James D. Rickman, Jr., a system for measuring the speed of a turbomachine through the fixed housing of the machine is disclosed. The basic principle of the transducer used by Rickman was disclosed earlier by Louis Rosenthal in U.S. Pat. No. 3,065,412. Rosenthal pointed out that the amplitude of the signal induced in a coil by a conductive object passing through the static magnetic field of the invention is influenced by many factors; particle size, flux density, coil turns, particle permeability, air gap spacing and velocity. He did not teach how to use these signals to measure anything about the particle, probably because of the number of factors influencing the signals and their complex inter-relationships. Rickman uses only the frequency of the signal to measure speed of the turbomachine rotor. Others who have used the configuration of Rosenthal for conductive article detection, without making further use of information inherent in the signals, are Roy A. Applequist (U.S. Pat. No. 3,530,323) and William L. Garrott (U.S. Pat. Nos. 3,896,608 and 3,964,042).
Judging from the activity in this field, i.e. the number of types of transducers which have been developed and tried, it is apparent that measurement of turbomachinery blade conditions during machine operation, in a manner suitable for long term monitoring, is an important and unsolved problem.